Cape Breton Highlands National Park of Canada

Periwinkles - A Common Seashore Find

Periwinkles are a common intertidal invertebrate on rocky shores. Here, common and smooth periwinkles cling to a rock, waiting for high tide to return.
© Cape Breton Highlands National Park / B. MacDonald

Periwinkles are marine snails. They have spiraled snail shells and the soft-bodied snail itself is dark grey with two tentacles on the head end at the base of which are its eyes. Periwinkles may be identified by differences in their shells. The common periwinkle is our largest periwinkle, with a spiraled shell usually about 2 centimetres long. The common periwinkle's shell is generally olive, grey or brown with dark brown bands.

The rough periwinkle and the common periwinkle are about equal in size (up to 1.3 centimetres long) but may be differentiated by the shape and colour of their shells. The rough periwinkle has a ridged spiral shell that is usually dark grey or brown and may sometimes be mottled with yellow and black. The smooth periwinkle's smooth, almost round spiral shell is the most brightly coloured, usually yellow, orange or brown with brown or white bands.

Distribution

Periwinkles are found on most rocky shores, especially where there are rocks covered with algae, which is what they eat. The common periwinkle lives on the lower beach in tidal pools and sometimes salt marshes. It is covered by water for longer periods of time than the other two. The smooth periwinkle lives on the middle beach in layers of rockweed. The rockweed stays damp when the tide goes out so the periwinkles do not dry out. The rough periwinkle lives on the upper beach and in salt marshes, feeding on algae when the tide is high. When the tide goes out, the rough periwinkle hides in shaded areas and crevices of rocks. It pulls its body into its shell and seals itself to the rock using mucus to prevent it from drying out.

More on Periwinkles

Periwinkles are food for groundfish, dog whelks and gulls. You can tell if the owner of an empty periwinkle shell was eaten by a dog whelk if there is a round hole bored through the shell. Periwinkles are edible for humans as well and are very popular as food in Europe. Large numbers of periwinkle shells have been found in old Aboriginal campsites along the Nova Scotian coast so it is likely that the Mi'kmaq liked eating them too.

The habitat of our three species of periwinkle affects how they reproduce. The common periwinkle, spending more time in water than the other two species, releases its eggs to float on the water. Its larvae hatch about 6 days later and swim about freely. The smooth periwinkle lays its eggs in jelly-like masses in the rockweed where it lives. The eggs hatch in about one week and the larvae swim in the water like the common periwinkle's. Rough periwinkles don't lay eggs because they spend so much time out of the water! Instead they keep the eggs in special pouches inside their shells until young snails emerge.

The common periwinkle was introduced to Nova Scotia from Europe over a century ago and within 30 years spread all over the Atlantic seaboard from Nova Scotia to New Jersey. Since then it has expanded its range even more and can be found from Labrador to Maryland. Although little or no economic impact was caused by the introduction of the common periwinkle, it has had an enormous impact on the ecosystem. It has fundamentally changed the distribution and abundance of algae on rocky shores, altered the interrelationships of organisms in several habitats, and negatively affected native molluscs such as the tortoiseshell limpet, the rough periwinkle and the mud dog whelk. It has also affected the hermit crab by providing more and larger shells for the crab to use.